Liquid crystal display device

ABSTRACT

It is an object of the present invention to provide a liquid crystal display device with a back-side prism type backlight in order to prevent a stain in an image displayed and attain high brightness. 
     A liquid crystal display device comprises: a first substrate; a second substrate arranged on the viewer side compared with the first substrate; a liquid crystal held between the first substrate and the second substrate; a light guide plate arranged on the back surface of the first substrate; a polarization plate arranged between the first substrate and the light guide plate; a prism sheet arranged between the light guide plate and the polarization plate and having prisms provided on the light guide plate side; a diffusion layer arranged between the prism sheet and the polarization plate; and a light control sheet arranged between the diffusion layer and the polarization plate. The light control sheet rotates the polarization of the light emitted from the diffusion layer.

CLAIM OF PRIORITY

The present application claims priority from Japanese Application JP2005-343580 filed on Nov. 29, 2005, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to liquid crystal display devices, andmore particularly to a technique suitable for application to back-sideprism type backlights.

2. Description of the Related Art

TFT (Thin Film Transistor) liquid crystal display modules with smallcolor liquid crystal display panels of about 240×320×3 subpixels arewidely used as display units in mobile appliances such as cell phones.

Such liquid crystal display modules for cell phones can be improved inbrightness by employing back-side prism type backlights.

FIG. 5 provides a cross sectional view of a main portion illustratingthe outlined structure of a conventional liquid crystal display modulefor cell phone, the module using a back-side prism type backlight.

As shown in FIG. 5, the conventional liquid crystal module for cellphone has a liquid crystal display panel and a back-side prism typebacklight which illuminates the liquid crystal display panel.

The backlight comprises: a light guide plate 6 having a nearlyrectangular shape similar to the plane shape of the liquid displaypanel; white light emitting diodes 8 disposed at one side surface(incident surface) of the light guide plate 6; a reflection sheet 7disposed on the bottom surface (opposite to the liquid crystal displaypanel side) of the light guide plate 6; a back-side prism sheet 5disposed on the top surface (liquid crystal display panel side) of thelight guide plate 6; and a diffusion sheet 4 disposed on the top surfaceof the back-side prism sheet 5. The back-side prism sheet 5 is a prismsheet having prisms on the side of the light guide plate 6.

The liquid crystal panel has a liquid crystal cell 2, an upperpolarization plate 1 attached to the top surface (display surface) ofthe liquid crystal cell 2 and a lower polarization plate 3 attached tothe bottom surface (backlight side) of the liquid crystal cell 2.

The technical documents related to the present invention include:

[Patent Document 1] Japanese Patent Laid-Open No. 2001-166116

[Patent Document 2] Japanese Patent Laid-Open No. 2001-166302

SUMMARY OF THE INVENTION

A back-side prism type backlight can efficiently obtain the frontluminance with use of refraction properties of prisms without diffusionby dots. However, the emitted light has polarization characteristics.

Due to the polarization characteristics, if it does not match thetransmission axis angle of the lower polarization plate 3 that isdisposed on the side of the backlight, the light emitted from thebacklight is absorbed by the lower polarization plate 3, resulting inlowering the transmittance of the liquid crystal display panel 1.

To solve this problem, use of a light control sheet which rotates thedirection of the maximum degree of polarization is described in PatentDocuments 1 and 2.

However, the conventional techniques described in the above PatentDocuments 1 and 2 have the following problems:

-   (1) In the case of Patent Document 2, since a diffusion sheet is not    used, it is difficult to suppress the in-plane unevenness of    luminance due to undulations of the prism sheet and the occurrence    of moire due to the angle of the arranging direction of prisms on    the prism sheet relative to that of pixels.-   (2) In the case of Patent Document 1, although the base sheet for    prisms is designed to have a function as a light control sheet,    adding a diffusion sheet reduces the effect of the light control    sheet since the birefringence of the diffusion sheet is not taken    into consideration. Since the diffusion sheet has birefringence    properties, the polarization axis of the transmitting light is    rotated by a different angle for each of R, G and B wavelengths.    Likewise, the diffusion sheet causes a different ellipticity of    polarization for each of the wavelengths. Therefore, since the    diffusion sheet is present between the light control sheet and the    lower polarization plate, the polarization of light is disturbed by    the diffusion sheet before the light enters the lower polarization    plate even if the light control sheet controls the polarization.    This reduces the efficiency of polarization control by the light    control sheet.-   (3) Since there is no consideration of the wavelength dependence of    the light before entering the light control sheet, images displayed    on the liquid crystal display panel may be stained.-   (4) Since there is no consideration of a method of fixing the light    control sheet, the light control sheet may have an undulation.

The present invention was made to solve the above-cited conventionaltechnical problems. It is an object of the present invention to providea technique that prevents a stain in an image displayed by a liquidcrystal display device with a back-side prism type backlight and thatattains high brightness in the image.

The above and other objects and novel features of the present inventionwill become apparent with reference to the description of the presentspecification and the accompanying drawings.

An outline of representative inventions disclosed in this specificationwill be briefly described below.

-   (1) A liquid crystal display device comprising: a first substrate; a    second substrate arranged on the viewer side compared with the first    substrate; a liquid crystal held between the first substrate and the    second substrate; a light guide plate arranged on the rear side    compared with the first substrate; a polarization plate arranged    between the first substrate and the light guide plate; a prism sheet    arranged between the light guide plate and the polarization plate    and having prisms on the light guide plate side; a diffusion layer    arranged between the prism sheet and the polarization plate; and a    light control sheet arranged between the diffusion layer and the    polarization plate and rotating the polarization of light emitted    from the diffusion layer.-   (2) A liquid crystal display device according to (1) wherein the    light control sheet rotates the polarization of the light emitted    from the diffusion layer so that the angle of the transmission axis    of the polarization plate relative to a straight line connected    between two points at which each of the R, G and B components of the    light emitted from the light diffusion layer has almost the same    amplitude in the polarization characteristics falls within the range    of ±10°.-   (3) A liquid crystal display device according to (2) wherein, the    following expressions are satisfied:    −0.03≦(A2−A1)/A1≦0.03 and −0.03≦(A3−A1)/A1≦0.03    where A1 is any one of the amplitudes of the R, G and B components,    and A2 and A3 are the amplitudes of the other two components at the    straight line.-   (4) A liquid crystal display device according to any one of (1)    through (3) wherein the light control sheet is a λ/2 retardation    plate.-   (5) A liquid crystal display device according to any one of (1)    through (4) wherein the light control sheet is arranged in contact    with the polarization plate.

The following provides a brief description of effects which can beattained by the representative inventions disclosed in thisspecification:

According to the present invention, liquid crystal display devices thateach use a back-side prism type backlight can prevent a stain in animage displayed while attaining high brightness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective diagram schematically showing thestructure of a liquid crystal display module for cell phone according toan embodiment of the present invention.

FIG. 2 is a cross sectional view of the main part schematically showingthe structure of the liquid crystal display module for cell phoneaccording to the embodiment of the present invention.

FIG. 3 is a schematic diagram to show how polarization characteristicsof R, G and B components of light emitted from the diffusion sheetvaries in the present liquid crystal display module according to theembodiment of the present invention.

FIG. 4 shows the relationship between the transmittance (F) of the lowerpolarization plate and the slow axis angle (θd) of the λ/2 retardationplate, where the light control sheet shown in FIG. 2 is composed of aλ/2 retardation plate.

FIG. 5 provides a cross sectional view of the main part schematicallyshowing a conventional liquid crystal display module for cell phone,which uses a back-side prism type backlight.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention will be described below in detailwith reference to the drawings.

Each of components having the same function is given the same numeralreference in all the drawings to explain the embodiment to avoid anunnecessary repetition.

Embodiment

FIG. 1 is an exploded perspective diagram which schematically shows thestructure of a liquid crystal display module for cell phone according tothe embodiment of the present invention. FIG. 2 is a cross sectionalview of the main part schematically showing the structure of the liquidcrystal display module for cell phone according to the embodiment of thepresent invention.

As shown in FIGS. 1 and 2, like the abovementioned conventional liquidcrystal display module, the liquid crystal module according to theembodiment of the present invention has a liquid crystal display panel10 and a back-side prism type backlight (B/L) for illuminating theliquid crystal display panel 10.

The backlight (B/L) includes: a light guide plate 6 having asubstantially rectangular shape similar to the plane shape of the liquiddisplay panel 10; white light emitting diodes (light sources) 8 disposedat one side surface (incident surface) of the light guide plate 6; areflection sheet 7 disposed on the bottom surface (opposite to theliquid crystal display panel side) of the light guide plate 6; aback-side prism sheet 5 disposed on the top surface (liquid crystaldisplay panel side) of the light guide plate 6; a diffusion sheet(diffusion layer) 4 disposed on the top surface of the back-side prismsheet 5; and a resin frame 20. The back-side prism sheet 5 has prismsprovided on side of the light guide plate 6. The diffusion sheet 4 maybe replaced with a diffusion layer formed on the top surface of theback-side prism sheet 5 by means of coating although the diffusion sheet4 is used in the description of the present embodiment.

The present liquid crystal display module according to the embodimenthas the diffusion sheet 4, the back-side prism sheet 5, the light guideplate 6 and the white light emitting diodes 8 arranged in the resinframe 20 in this order, shown in FIG. 1. The reflection sheet 7 isarranged on the bottom surface of the resin frame 20.

The liquid crystal display panel 10 includes: a liquid crystal cell 2;an upper polarization plate 1 attached to the top surface (displaysurface) of the liquid crystal cell 2; and a lower polarization plate 3attached to the bottom surface (backlight side) of the liquid crystalcell 2. Although not shown in the figure, a retardation plate may beincluded between the upper polarization plate 1 and the liquid crystalcell 2. In addition, a retardation plate (not shown) may be includedbetween the lower polarization plate 3 and the liquid crystal cell 2.

The liquid cell 2 comprises a glass substrate (or called a TFTsubstrate) 2 b and a glass substrate (or called a opposite substrate).On the glass substrate 2 b, pixel electrodes, thin film transistors andthe like are formed. Color filters and the like are formed on the glasssubstrate 2 a. To construct the liquid crystal cell 2, the twosubstrates are overlapped with each other with a predetermined spaceleft between them and bonded to each other by means of a frame-shapedsealing material formed near the edges between the substrates. Afterthat, liquid crystal is filled into the sealing material placed betweenthe substrates from a liquid crystal inlet formed in a portion of thesealing material, and the inlet is sealed. In addition, a semiconductorchip (DRV) which includes a driver and the like is mounted on the glasssubstrate 2 b.

Note that the resin frame 20 and the semiconductor chip (DRV) areomitted in FIGS. 2 and 5. Further, although a flexible wiring substrateis mouthed on the glass substrate 2 b in order to supply control signalsand the like to the semiconductor chip (DRV), this flexible wiringsubstrate is omitted in FIGS. 2 and 5.

The present embodiment is different from the abovementioned conventionalliquid crystal display module in that a light control sheet 9 isarranged between the lower polarization plate 3 and the diffusion sheet4. Although the light control sheet 9 is preferably made in contact withthe lower polarization plate 3, the present invention is not limited tothis arrangement. To arrange the light control sheet 9 in contact withthe lower polarization plate 3, an adhesive is preferably used the lightcontrol sheet 9 rotates the polarization (polarization direction) of thelight emitted from the backlight (B/L) before the light enters the lowerpolarization plate 3.

FIG. 3 is a schematic diagram to indicate how the polarizationcharacteristics of the R, G and B components of the light emitted fromthe diffusion sheet 4 varies in the liquid crystal display moduleaccording to the embodiment of the present invention.

In FIG. 3, the angle is measured counterclockwise with the horizontaldirection of the screen (left-right direction in the figure) as thereference (0°).

In FIG. 3, R, G and B indicate the polarization characteristics of thered (R), green (G) and blue (B) components of the light, respectively.

As shown in FIG. 3, each of the polarization characteristics of the R, Gand B components of the light emitted from the diffusion sheet 4 aresubstantially elliptical.

The light control sheet 9 rotates the polarization of the light emittedfrom the diffusion sheet 4.

However, the diffusion sheet 4 exhibits a birefringence dependent on thewavelength. Thus, each of the R, G and B components of the light emittedfrom the diffusion sheet 4 has a different direction of the maximumamplitude of polarization (the longitudinal axis direction of theellipse of each of the R, G and B components shown in FIG. 3).Therefore, images displayed on the display surface of the liquid crystaldisplay panel may be stained if the angle of rotation is notappropriately set.

In the present embodiment, the light control sheet 9 rotates thepolarization of the R, G and B components of the light emitted from thediffusion sheet 4 so that a straight line (P-P′ in FIG. 3) connectedwith two points at which each of R, G and B has the same amplitude matchthe transmission axis of the lower polarization plate 3. For example, ifthe transmission axis of the lower polarization plate 3 has an angle of165° in FIG. 3, the light control sheet 9 rotates the polarization ofthe light emitted from the diffusion sheet 4 by θ as shown in FIG. 3.

This equalizes the amplitudes of the R, B and G components incident onthe liquid crystal display panel 10. Therefore, a stain is preventedfrom being produced in a image displayed on the display surface of theliquid crystal display panel 10.

Note that the above description is the best case of the embodiment. Inpractical products, it is difficult to completely equalize theamplitudes. Thus, the light control sheet 9 maybe designed to rotate thepolarization of the light emitted from the diffusion sheet 4 so that theangle (narrower angle) of the transmission axis of the lowerpolarization plate 3 relative to the straight line (P-P′ in FIG. 3)connected with the two points at which each of R, G and B has the sameamplitude falls within the range of ±10° (preferably ±5° and morepreferably ±2.5°).

It is noted that the R, G and B components of the light emitted from thediffusion sheet 4 do not always have the same amplitude, in terms of thepolarization characteristics of the R, G and B components of the lightemitted from the diffusion sheet 4. Therefore, points where the R, G andB components have substantially the same amplitude are used while acertain degree of error (difference) is accepted. With respect to thestraight line (P-P′ in FIG. 3) connected with the points where the R, Gand B have substantially the same amplitudes, for example, the error maybe tolerable when the following expressions are satisfied:−0.03≦(A2−A1)/A1≦0.03 and −0.03≦(A3−A1)/A1≦0.03where, A1 is any one of amplitudes of the R, G and B components, and A2and A3 are the amplitudes of the other two components.

For example, the light control sheet 9 is composed of a λ/2 retardationplate made of cycloolefin-based material. FIG. 4 is a graph showing howthe transmittance (F) of the lower polarization plate 3 depends on theretardation axis angle (θd) of the λ/2 retardation plate that forms thelight control sheet 9. θd is measured as shown in FIG. 3.

From this graph, it is understood that the lower polarization plate 3can obtain high transmittance (F) when the retardation axis angle (θd)of the k/2 retardation plate satisfies the relation 7.5°≦θd≦27.5°. Thisfact means that the light control sheet 9 rotates the polarizationdirection P-P′ of FIG. 3 so that the angle of the P-P′ line placed afterthe rotation relative to the transmission axis of the lower polarizationplate 3 falls within the range of ±10°. In addition, FIG. 4 shows a peakvalue when 7.5°≦θd≦27.5°. This means that the light control sheet 9rotates the polarization direction P-P′ of FIG. 3 so that the angle ofthe P-P′ line placed after the rotation relative to the transmissionaxis of the lower polarization plate 3 falls within the range of ±2.5°.

The abovementioned Patent Document 1 describes that a stretched filmhaving birefringence is disposed to a back-side prism base sheet (32A),the polarization axis direction of the base sheet (32A) is set so thatthe polarization direction of the light which has passed through thebase sheet (32A) substantially match the polarization axis of the lowerpolarization plate and a diffusion sheet (40) is added as necessary.

According to Patent Document 1, however, since the diffusion sheet (40)is disposed above the base sheet (32A), polarization is disturbed due tothe birefringence of the diffusion sheet (40). This lowers the effectsof the base sheet (32A).

The abovementioned Patent Document 2 describes that the prism sheet(12), which is a mono-axially stretched film or uni-axially stretchedfilm, rotates the direction of the maximum degree of polarization of thelight emitted from the light guide plate (4) toward the transmissionaxis direction of the polarization plate (14) in order to increase theamount of light of polarization components passing through thepolarization plate, and a separate light control sheet to rotatepolarization is also provided.

According to Patent Document 2, however, no diffusion sheet is usedbetween the backlight (B/L) and the liquid crystal display panel 10 toprevent optical interference. In this configuration, it is possible thatoptical interference (moiré) may occur, making it impossible to attainuniform effects in the in-plane direction.

In the case of the present embodiment, on the other hand, since thelight control sheet 9 is arranged between the diffusion sheet 4 and thelower polarization plate 3, the light incident on the lower polarizationplate 3 can be rotated in consideration of the birefringence of thediffusion sheet (40). Therefore, even if the polarization is disturbeddue to the birefringence of the diffusion sheet (40), the effects of thelight control sheet 9 are not reduced. Further, the diffusion sheet 4can suppress the occurrence of moiré.

In addition, in Patent Documents 1 and 2, since the lower polarizationplate and the light control sheet are separately arranged, it ispossible that conditions for obtaining the maximum intensity of lightmay not be stably attained due to an assembly misalignment.

Further, in Patent Documents 1 and 2, since the surface of the lightcontrol sheet is not fixed, the light control sheet may have anundulation. If the light control sheet is pressed by another sheet toprevent such an undulation, moire may occur due to interference withthis sheet.

If a diffusion sheet is added on the light control sheet to prevent amoire pattern, the control performance of the light control sheet may bedeteriorated. It is not possible to attain uniform effects in thein-plane direction.

In the case of the present embodiment, the light control sheet 9 isarranged in contact with the lower polarization plate 3. This can reducethe influence of the assembly misalignment between the lower polarizersheet 3 and the light control sheet 9 and prevent the light controlsheet 9 from having an undulation.

While the present invention has been described above in detail based onthe embodiment thereof, it is to be understood that the presentinvention is not limited to the embodiment and various modifications maybe made thereto without departing from the spirit of the invention.

1. A liquid crystal display device comprising: a first substrate; asecond substrate arranged on the viewer side compared with the firstsubstrate; a liquid crystal held between the first substrate and thesecond substrate; a light guide plate arranged on the back surface ofthe first substrate; a polarization plate arranged between the firstsubstrate and the light guide plate; a prism sheet arranged between thelight guide plate and the polarization plate and having prisms providedon the light guide plate side; a diffusion layer arranged between theprism sheet and the polarization plate; and a light control sheetarranged between the diffusion layer and the polarization plate; whereinthe light control sheet rotates the polarization of the light emittedfrom the diffusion layer.
 2. A liquid crystal display device accordingto claim 1, wherein the light control sheet rotates the polarization ofthe light emitted from the diffusion layer so that the angle of thetransmission axis of the polarization plate relative to a straight lineconnected with points where each of R, G and B components of the lightemitted from the light diffusion layer has substantially the sameamplitude in the polarization characteristics falls within the range of±10°.
 3. A liquid crystal display device according to claim 2, whereinthe following expressions are satisfied:−0.03≦(A2−A1)/A1≦0.03 and −0.03≦(A3−A1)/A1≦0.03 where, A1 is any one ofamplitudes of the R, G and B components, and A2 and A3 are theamplitudes-of the other components at the straight line.
 4. A liquidcrystal display device according to claim 1, wherein the light controlsheet is a λ/2 retardation plate.
 5. A liquid crystal display deviceaccording to claim 1, wherein the light control sheet is arranged incontact with the polarization plate.
 6. A liquid crystal display devicecomprising: a first substrate; a second substrate arranged on the viewerside compared with the first substrate; a liquid crystal held betweenthe first substrate and the second substrate; a light guide platearranged on the back surface of the first substrate; a polarizationplate arranged between the first substrate and the light guide plate; aprism sheet arranged between the light guide plate and the polarizationplate and having prisms provided on the light guide plate side; adiffusion sheet arranged between the prism sheet and the polarizationplate; and a light control sheet arranged between the diffusion sheetand the polarization plate; wherein the light control sheet rotates thepolarization of the light emitted from the diffusion sheet so that theangle of the transmission axis of the polarization plate relative to astraight line connected with points where each of R, G and B componentsof the light emitted from the diffusion sheet has substantially the sameamplitude in the polarization characteristics falls within the range of±10°.
 7. A liquid crystal display device according to claim 6, whereinthe following expressions are satisfied:−0.03≦(A2−A1)/A1≦0.03 and −0.03≦(A3−A1)/A1≦0.03 where, A1 is any one ofthe amplitudes of the R, G and B components, and A2 and A3 are theamplitudes of the other components at the straight line.
 8. A liquidcrystal display device according to claim 6, wherein the light controlsheet is a λ/2 retardation plate.
 9. A liquid crystal display deviceaccording to claim 6, wherein the light control sheet is arranged incontact with the polarization plate.
 10. A liquid crystal display devicecomprising: a first substrate; a second substrate arranged on the viewerside compared with the first substrate; a liquid crystal held betweenthe first substrate and the second substrate; a light guide platearranged on the back surface of the first substrate; a polarizationplate arranged between the first substrate and the light guide plate; aprism sheet arranged between the light guide plate and the polarizationplate and having prisms provided on the light guide plate side thereof;a diffusion layer arranged between the prism sheet and the polarizationplate; and a light control sheet arranged between the diffusion layerand the polarization plate; wherein the light control sheet is arrangedin contact with the polarization plate and rotates the polarization ofthe light emitted from the diffusion layer.